Three weeks ago I found a stone on Gemstone Beach (stone A below) that could be a hydrogrossular garnet. But perhaps it could be chalcedony instead. It had a rough coating that sometimes is seen on chalcedony. Although chalcedony is rare on Gemstone Beach, I have found a piece nearby (at McCracken’s Rest). However, the stone has a faint green hue that is often seen in hydrogrossular and it feels especially heavy, another characteristic of hydrogrossular. In our recent book on Gemstone Beach, I included in the hydrogrossular section an example of a similar stone (page 23, stone #44) which Nick Mortimer noted could instead be chalcedony. Having seen how Michelle Harrington had tested some Gemstone Beach stones recently for their specific gravity, I decided to do this myself to see if it would throw light on what my find might be.
TESTING FOR SPECIFIC GRAVITY
I don’t fully understand all the physics involved, but I followed this displaced water procedure: I weighed the dry stone on a good digital scale that gives measures to the gram, then dangled it in water using cotton without touching the bottom or sides of the container, measuring the increase in the weight of the container and water that resulted. The dry weight is then divided by the displaced water weight to give the specific gravity. This is more fully explained in this video:
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THE SIX STONES TESTED FOR THEIR SPECIFIC GRAVITY
I weighed six stones:
Stone A is the recently-found stone in question and stone B is stone #44 from page 23 in the book (found on Gemstone Beach in April 2023) – both could be either hydrogrossular or chalcedony:
Stones C and D are hydrogrossular garnets I found on Gemstone Beach – stone C was found in March 2022 and stone D was found on 26 February 2021:
Stone E is another Gemstone Beach stone that I suspect is vesuvianite (also known as idocrase) which appears in the book on page 23 as stone #43:
Vesuvianite frequently looks similar to hydrogrossular garnet, shares a similar crystal structure, and often grows in the same metamorphic rock environments alongside grossular garnet (What is Vesuvianite). It is said to have a vitreous and resinous lustre (Mindat), which characterises stone E and sets it apart from the hydrogrossular garnets I tested. And Stone F is a piece of botryoidal chalcedony that I had found near Moeraki village a few years ago – see the entry for Day 20, Monday 8 March 2021 in this Post. It is the purest piece of chalcedony in my collection.
The specific gravity of chalcedony is standardised at 2.6 (see Mindat). As I noted when describing Michelle’s testing, there are conflicting views on the specific gravity of hydrogrossular garnet.
WHAT IS THE SPECIFIC GRAVITY OF HYDROGROSSULAR GARNET?
Wikipedia gives hydrogrossular’s secific gravity as 4.15 but a lot of other sources give it more around 3.5. For a while I considered the specific gravity of similar garnet varieties. When you look for hydrogrossular in Mindat, you find the following in the hibschite entry: “Hibschite is now an informal intermediate composition in the Grossular-Katoite Series, mostly previously known as hydrogrossular”. The specific gravity of hibschite is given there as 3.06-3.25 while the specific gravity in Mindat of katoite is calculated as 3.013. AlexStrekeisen notes that hydrogrossular is at the grossular end of the grossular-katoite series and Mindat gives grossular garnet’s specific gravity as 3.6. These observations place hydrogrossular as much closer to 3.5 than 4.15.
Then I dug a little deeper into Wikipedia’s view of hydrogrossular’s specific gravity. It notes that its source is the Gemological Institute of America (GIA) – I searched for the GIA Gem Reference Guide and was able to access it here. Making my way through an attack of pop-up ads, found that the specific gravity of hydrogrossular is recorded as 3.47 (so I don’t know where Wikipedia got 4.15). I also found an article on the GIA website that had analysed two bangles claiming to be jadeite but proved to be hydrogrossular, their specific gravity being determined as 3.42 and 3.45 – using the Reference Guide figure. When I asked Google for hydrogrossular’s specific gravity, its AI Overview stated: “The specific gravity of hydrogrossular garnet typically ranges from 3.35 to 3.55 (with broader or mineralogical reference ranges spanning from 3.25 to 4.15 depending on the exact degree of hydroxyl (OH) substitution for silica [SiO₄]).” I have checked the references given for the AI Overview but did not find what any that could be referred to as “mineralogical sources”. I have consulted a number of my general texts on rocks and minerals and they don’t refer to hydrogrossular though they have entries for grossular garnet which agreeing on 3.6.
In conclusion, I believe we can say that the specific gravity of hydrogrossular garnet is certainly greater than that of chalcedony and could be expected to be around 3.5. Of course, the presence of other minerals in a stone can make the specific gravity a little higher or lower.
TESTING RESULTS
My testing calculated the botryoidal chalcedony (labelled F) to have a specific gravity of 2.57 – dry weight was 54 grams and water displacement was 21 grams. So it is pratically spot on what we would expect for chalcedony. The two hydrogrossular garnets labelled C and D respectively had specific gravities of 3.5 (dry=144 grams, water displacement=41 grams) and 3.36 (dry=205 grams, water displacement=61 grams) – both noticeably higher than the chalcedony and very close to what is expected for hydrogrossular. My recent find (labelled A) has a dry weight of 386 grams and water displacement of 113 grams, giving a specific gravity of 3.42, demonstrating it is not chalcedony and showing a consistency with hydrogrossular (though of course not providing that as a certain identification). Stone B, with similar visual characteristics to stone A, turned out to have a specific gravity of 3.48 (dry=80 grams, water displacement=23 grams), very similar to that of stone A, meaning it too is not chalcedony. Stone E, which I suspect to be vesuvianite, has a specific gravity of 3.5 (dry = 95 grams, water displacement=27 grams) – Mindat records its specific gravity as 3.32-3.43, just a tad lower than hydrogrossular.
Taking everything into account (including its visual characteristics and weight), I am now happy that my recent find, and others similar to it from Gemstone Beach, is highly likely to be hydrogrossular garnet.
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